I have the same skimmer. At first, it was in a 20 gallon sump for my 55 fowlr. Every once in a while, especially when the auto top-off kicked in, I would find a huge puddly mess on the floor. When the fresh water would get sucked in, it would just overflow the collection cup and spill over the sides of the 20 gallon sump. Every time I did a water change, I would have to constantly readjust it and come back in about an hour to see how it was doing.
Since I upgraded to a 125 fowlr with a 30 gallon sump with baffles, I never have to touch the thing. If the top-off seems to be your problem, I'd try adding baffles. I don't know how big your sump is, but if you only have a 75 gallon tank, I can't imagine it being very big.
Good Luck.